My college physics building had a lab where the individual experiment rooms had light-blocking door sweeps and blackout curtains on the windows. I didn't need a dark room -- I don't know what experiments needed that actually -- but I pulled the curtain to see just how dark it could get -- very dark was the answer. But when my eyes adjusted a bit, on the wall opposite the window was the outside world, upside down! There was a tiny pinhole fissure in the blackout curtain! I'd read of camera obscura, so it wasn't a total surprise to me, but it was fun to see the effect for real and in person.